This invention relates to methods of manufacturing ferroelectric and antiferroelectric liquid crystal devices, and is concerned more particularly with the formation of at least one alignment layer contacting a layer of liquid crystal material within the device.
The surface stabilised ferroelectric liquid crystal device (SSFLCD) possesses the advantage over other liquid crystal devices, such as the twisted nematic liquid crystal device, that it is a bistable device which can be switched between two states by switching pulses of alternate polarity and which will remain in one state in the absence of a switching pulse until a switching pulse of opposite polarity is applied to switch it to the opposite state. By contrast, in operation of a twisted nematic liquid crystal device, a drive signal must be applied continuously to maintain the device in one of its states. It is also a particular advantage of SSFLCD""s that the individual switching elements can be passively addressed without requiring an active matrix in which a respective switching transistor is associated with each switching element.
As is well known, a conventional SSFLCD typically includes a cell in which a layer of chiral smectic ferroelectric liquid crystal material is contained between two parallel transparent substrates provided on their inside surfaces with electrode structures which may be in the form of row and column electrode tracks crossing one another to form an addressable matrix of switching elements utilising different areas of the liquid crystal material. Such switching elements are typically addressed on a line-by-line basis by applying data pulses in parallel to the column electrode tracks, each data pulse being either a switching pulse or a non-switching pulse, and by applying strobe pulses to the row electrode tracks so as to switch selected switching elements along each row from one state to the other under the effect of the electric field produced by the voltage difference between the data pulse and the strobe pulse applied to the relevant electrode tracks.
Furthermore rubbed alignment layers may be provided on the inside faces of the substrates in order to impart a preferred alignment to the molecules of the liquid crystal material in the vicinity of the alignment layers. In general this leads to the liquid crystal molecules being uniformly aligned in microlayers extending perpendicularly to the substrates with the molecules in each microlayer adopting a chevron geometry due to the alignment of the molecules with the alignment layers on both sides of the liquid crystal layer. However it should be understood that such a device structure is only one possible example of different structures of liquid crystal device to which the invention is applicable, and furthermore the particular addressing arrangement described is given as only one example of addressing arrangement which may be used in such a device.
Furthermore, in certain applications, it is possible to arrange for partial switching of the switching elements of the device by means of a switching signal of reduced voltage and/or reduced duration so that only a part of the area of the liquid crystal material corresponding to the switching element changes state. In the case where the device is a display device and the two fully-switched states of each switching element correspond to black and white states, such partial switching can be used to provide analogue greyscale. However the control of greyscale is rendered difficult by the problem of controlling domain formation during partial switching of the device.
A number of proposals have previously been made for controlling domain formation in a ferroelectric liquid crystal device. For example, JP 03048819 (Matsushita) and JP 04127124 (Asahi Glass) disclose techniques for fabricating arrays of microstructures within the device to provide nucleation points for controlling domain formation within the liquid crystal material. However such techniques require extra cell fabrication steps which lead to increased manufacturing complexity and cost.
Furthermore E. Matsui and A. Yasuda, FLC 95 Abstracts (1995) 97-99 and EP 0595219A (Sony) disclose techniques in which small balls are distributed in the liquid crystal material to act as nucleation points for controlling domain formation. However it is difficult to obtain good uniformity of distribution of such balls within the liquid crystal material. JP 194635/1994 and EP 0586014A (Phillips) disclose techniques for forming a structure in which non-reactive chiral liquid crystal molecules are captured in an anisotropic 3-D network structure made of a polymeric material. In this technique the network structure stabilises microscopic domains having opposite polarisation directions, so that greyscale can be maintained even after the applied electric field has been removed. However, because of the relatively high concentration of polymer which remains within the liquid crystal material after manufacture, the viscosity of the liquid crystal material is increased and this tends to lead to slower switching.
Furthermore European Patent Application No. 97309839.5 discloses a technique in which a dopant is mixed with the liquid crystal material, and the dopant is then caused to separate out of the liquid crystal material as a result of a change in the processing conditions, such as a change in temperature or application of ultraviolet light, which preferably results in a change in phase of the dopant so as to provide nucleation points for controlling domain formation within the liquid crystal material.
D. C. Ulrich, M. J. Cherrill and S. J. Elston, xe2x80x9cSurface Modification and the Switching Processes in Ferroelectric Liquid Crystalsxe2x80x9d, Liquid Crystals, 1997, Vol. 23, No. 6, 797-802 discusses the manner in which switching of ferroelectric liquid crystal devices is affected by inherent seed sites, such as spacer beads and dirt particles, for example, and proposes defect seeding by deliberate introduction of seed features during manufacture of a device. In the particular example investigated in this paper, seed points and lines are produced by depositing a layer of photoresist of about 0.5 xcexcm thickness on the inside surface of one of the substrates of the device, the layer being subsequently exposed through a suitable mask, developed and hard baked to produce the seed points and/or lines, prior to coating with the alignment layer. However such a method involves a significant number of additional manufacturing steps, and thus increases both the complexity and the cost of production.
EP 0732610A2 discloses the use of an alignment layer comprising two or more types of polymer in a twisted nematic (TN) liquid crystal display device in order to improve contrast and widen the viewing angle of the device. The difference between the maximum SP (solubility parameter) value and the minimum SP value of the polymers used is at least 1. Preferably at least one of the polymers is a polymer containing siloxane or fluorine. In this manner it can be ensured that the alignment layer is capable of forming two or more different alignment states, so that two or more liquid crystal alignment states, for which the pretilt angle of the liquid crystal molecules differs, are present in the device. It is therefore ensured that the viewing angle characteristics are widened as compared with a similar device having only a single liquid crystal alignment state. However such an alignment layer is not suitable for improving the partial switching behaviour of a ferroelectric liquid crystal device.
It is an object of the invention to provide a ferroelectric or antiferroelectric liquid crystal device manufacturing method which can be used to improve the partial switching properties of the device.
According to the present invention there is provided a method of manufacturing a ferroelectric or antiferroelectric liquid crystal device in order to improve its partial switching behaviour, the device comprising a layer of ferroelectric or antiferroelectric liquid crystal material contacted by at least one alignment layer, wherein the alignment layer is formed by deposition on a substrate of at least two different alignment solutions having properties such that, when the solutions are brought together, they form areas having different alignment characteristics relative to the liquid crystal material which serve to control switching of molecules of the liquid crystal material between different alignment states in response to an applied electric field in order to permit partial switching of the device.
The areas of different alignment properties within the alignment layer formed in this manner interact differently with different regions of the liquid crystal material so that, by suitable control of the sizes and shapes of the different areas, it is possible to control the switching behaviour of the liquid crystal material in response to an applied electric field. In particular it is possible to effect control of the partial switching behaviour of the device, and to thereby control the analogue greyscale in a display device which relies on such partial switching for the display of grey levels.
The manner in which the areas of different alignment properties interact with the liquid crystal material may differ in different devices produced by the method of the invention. For example, the alignment solutions may contain materials having different interaction energies relevant to the liquid crystal material, in which case the switching behaviour of the liquid crystal material will be determined by the different anchoring properties of the alignment materials. Additionally or alternatively the alignment solutions may contain materials having different dielectric permittivities, in which case the switching properties of the liquid crystal material will be modified by the different voltage drops across the alignment materials.
Furthermore the areas of different alignment properties may form a pattern in which some areas are raised relative to other areas of the pattern so as to impart a rough surface to the alignment layer.
Such a method can be used to produce different surface features which serve as nucleation sites to control domain formation during switching of the liquid crystal material. Such surface features can be either small round features which will tend to act as nucleation points to speed up switching or alternatively elongate features which will tend to act as pinning points which slow down switching. A combination of these features may be found in the same alignment layer.
Furthermore, in order to ensure that the alignment materials form areas within the alignment layer having different alignment properties, either the alignment materials themselves are immiscible relative to one another, or alternatively the alignment materials are deposited on the substrate within respective solvents which are immiscible relative to one another, the solvents subsequently being evaporated leaving the alignment materials forming the required areas of the alignment layer.
Furthermore at least one of the alignment solutions may be mixed with a surfactant prior to deposition on the substrate so as to modify the wetting of the substrate by the solution. This can be used to produce surface features which again result in differential alignment properties serving to control switching of the liquid crystal material between alignment states in response to an applied electric field.
The invention also provides a ferroelectric or antiferroelectric liquid crystal device having a layer of ferroelectric or antiferroelectric liquid crystal material contacted by at least one alignment layer, wherein the alignment layer contains areas formed by deposition of at least two different alignment solutions on a substrate, said areas comprising polymers having different alignment characteristics relative to the liquid crystal material which serve to control switching of the molecules of the liquid crystal material between different alignment states in response to an applied electric field in order to permit partial switching of the device.